Special Senses

The Senses General senses

Temperature (cold &heat)

Pressure

Fine touch

Pain

Proprioceptors of muscles and joints

Special senses

Smell - Taste

Sight - Hearing

Equilibrium

The Eye and Vision

70 percent of all sensory receptors are in the eyes

Each eye has over a million of nerve fibers

Protection for the eye

Most of the eye is enclosed in a bony orbit(1/6 of the eye surface is seen)

A cushion of fat surrounds most of the eye

Accessory structures of the eye Externsic eye muscles

Eyelids

Conjunctiva

Lacrimal apparatus

Accessory Structures of the Eye

1-Eyelids and

Eyelashes

Figure 8.1b

Meibomian glands(associated wih eyelid edges )– modified sebacious glands produce an oily secretion to lubricate the eye

Figure 8.1b

Ciliary glands – modified sweat glands between the eyelashes

Figure 8.1b

2-Conjunctiva

Membrane that lines the eyelids

Covers part of the outer surface of the eyeball ,ends at the edge of the cornea

Secretes mucus to lubricate the eye

3-Lacrimalapparatus

Lacrimal gland – produces lacrimal fluid

Lacrimal canals – drains lacrimal fluid from eyes

Figure 8.1a

Lacrimal sac – provides passage of lacrimal fluid towards nasal cavity

Figure 8.1a

Accessory Structures of the Eye Nasolacrimal duct – empties lacrimal

fluid into the nasal cavity

Figure 8.1a

Function of the Lacrimal Apparatus

Properties of lacrimal fluid

Dilute salt solution (tears)

Contains antibodies and lysozyme

Protects, moistens, and lubricates the eye

Empties into the nasal cavity

4-Extrinsic Eye Muscles

Muscles attach to the outer surface of the eye

Produce eye movements

Figure 8.2

Structure of the Eye The wall is composed of three tunics

Fibrous tunic – outside layer

Choroid – middle layer

Sensory tunic – inside layer

Figure 8.3a

The Fibrous Tunic Sclera

White connective tissue layer

Seen anteriorly as the “white of the eye”

Cornea(many nerve ending,no blood vessels)

Transparent, central anterior portion

Allows for light to pass through

Repairs itself easily

The only human tissue that can be transplanted without fear of rejection

Choroid Layer

Blood-rich nutritive tunic

Dark pigment prevents light from scattering

Modified interiorly into two structures

Cilliary body – smooth muscle

Iris

Pigmented layer that gives eye color

Pupil – rounded opening in the iris

Sensory Tunic (Retina) Contains receptor cells (photoreceptors)

Rods

Cones

Signals pass from photoreceptors via a two-neuron chain

Bipolar neurons

Ganglion cells

Signals leave the retina toward the brain through the optic nerve.

Neurons of the Retina

Figure 8.4

Neurons of the Retina and Vision

Rods

Most are found towards the edges of the retina

Allow dim light vision and peripheral vision

Perception is all in gray tones

Neurons of the Retina and Vision

Cones

Allow for detailed color vision

Densest in the center of the retina

Fovea centralis – area of the retina with only cones,lateral to blind spot

It is the area of greatest visual acuity.

No photoreceptor cells are at the optic disk, ( blind spot)

Cone Sensitivity There are three

types of cones

Different cones are sensitive to different wavelengths

Color blindness is the result of lack of one cone type

Figure 8.6

Lens Biconvex crystal-like structure

Held in place by a suspensory ligament attached to the ciliary body

Figure 8.3a

Internal Eye Chamber Fluids

Aqueous humor

Watery fluid found in chamber between the lens and cornea

Similar to blood plasma

Helps maintain intraocular pressure

Provides nutrients for the lens and cornea

Reabsorbed into venous blood through the canal of Schlemm

Vitreous humor

Gel-like substance behind the lens

Keeps the eye from collapsing

Lasts a lifetime and is not replaced

Lens Accommodation Light must be

focused to a point on the retina for optimal vision

The eye is set for distance vision (over 20 ft away)

The lens must change shape to focus for closer objects

Figure 8.9

Images Formed on the Retina

Figure 8.10

Visual Pathway

Photoreceptors of the retina

Optic nerve

Optic nerve crosses at the optic chiasma

Figure 8.11

Visual Pathway

Optic tracts

Thalamus (axons form optic radiation)

Visual cortex of the occipital lobe

Figure 8.11

Eye Reflexes

Internal muscles are controlled by the autonomic nervous system

Bright light causes pupils to constrict through action of radial and circular muscles of iris

Viewing close objects causes accommodation

External muscles control eye movement to follow objects

Viewing close objects causes convergence (eyes moving medially)

The Ear

Houses two senses

Hearing

Equilibrium (balance)

Receptors are mechanoreceptors

Different organs house receptors for each sense

Anatomy of the Ear

The ear is divided into three areas

Outer (external) ear

Middle ear

Inner ear

Figure 8.12

The External Ear Involved in hearing only

Structures of the external ear

Pinna (auricle)

External auditory

canal

Figure 8.12

The External Auditory Canal

Narrow chamber in the temporal bone

Lined with skin

Ceruminous (wax) glands are present

Ends at the tympanic membrane

The Middle Ear or Tympanic Cavity

Air-filled cavity within the temporal bone

Only involved in the sense of hearing

The Middle Ear or Tympanic Cavity

Two tubes are associated with the inner ear

The opening from the auditory canal is covered by the tympanic membrane

The auditory tube connecting the middle ear with the throat

Allows for equalizing pressure during yawning or swallowing

This tube is otherwise collapsed

Bones of the Tympanic Cavity

Three bones span the cavity

Malleus (hammer)

Incus (anvil)

Stapes (stirrip)

Figure 8.12

Bones of the Tympanic Cavity

Vibrations from eardrum move the malleus

These bones transfer sound to the inner ear

Figure 8.12

Inner Ear or Bony Labyrinth

Includes sense organs for hearing and balance

Filled with perilymph

Figure 8.12

Inner Ear or Bony Labyrinth

A maze of bony chambers within the temporal bone

Cochlea

Vestibule

Semicircular canals

Figure 8.12

Organs of Equilibrium Receptor cells are in two structures

Vestibule for Static equilibrium

Semicircular canals for Dynamic equilibrium

Figure 8.14a–b

Chemical Senses – Taste and Smell

Both senses use chemoreceptors

Stimulated by chemicals in solution

Taste has four types of receptors

Smell can differentiate a large range of chemicals

Both senses complement each other and respond to many of the same stimuli

Olfaction – The Sense of Smell

Olfactory receptors are in the roof of the nasal cavity

Neurons with long cilia

Chemicals must be dissolved in mucus for detection

Impulses are transmitted via the olfactory nerve

Interpretation of smells is made in the cortex

Olfactory Epithelium

Figure 8.17

The Sense of Taste The tongue is covered

with projections called papillae . Taste buds are found on the sides of the papillae.

Location of taste buds:

On the tongue(most)

On Soft palate

On Cheeks

*Gustatory (taste) cells are present in the buds Figure 8.18a–b

Structure of Taste Buds

Impulses are carried from the gustatory cells to the brain by several cranial nerves because taste buds are found in different areas:

Facial nerve

Glossopharyngeal nerve

Vagus nerve

Anatomy of Taste Buds

Figure 8.18

Developmental Aspects of the Special Senses

Formed early in embryonic development

Eyes are outgrowths of the brain

All special senses are functional at birth